Troop and vehicle movements induce ground vibrations, with attendant weak seismic signals emanating from the location of such activity. These seismic signals are always produced at some level, and can be converted to valuable information for the detection and monitoring of movements within an area of interest when the monitoring technique is sufficiently sensitive. The prior art devices are unable to detect small seismic signals. The prior art devices are able to detect relatively large seismic signals from moving vehicles or perhaps large troop formations, but today's seismic detector cannot detect an individual enemy soldier or terrorist sneaking up on tiptoe to attack a perimeter guard. The prior art seismic detectors cannot detect small seismic signals because they generally lack adequate sensitivity. Prior art seismic detectors can adequately detect and measure seismic activity at strain values of about 10μ. Up until now, there is no available seismic detector that can detect and measure at the lower strain values of below 1.0 μ.
Another problem with prior art seismic detecting equipment has been the placement of the seismic sensor in such a way that the user is not observed or detected by the opposing forces. When prior art seismic sensors are placed in an array with a group of other sensors such an arrangement typically requires individual wire or wireless links between the detectors and a control station. Linking prior art detectors in this way suffers from a number of disadvantages, limitations and shortcomings including the equipment cost and a detectable signature, particularly when wireless links are used, along with difficulties caused by lack of adequate connector reliability. Thus, there has been a long-felt need for a seismic detector that can detect and measure at the lower strain values of about 1.0μ that does not suffer from the prior art's disadvantages, limitations and shortcomings of high equipment cost, detectable signature and lack of adequate connector reliability. Needless to say, if the seismic signals can be identified accurately in a remote and real-time mode, this would greatly aid intelligence gathering, battlefield monitoring and battle planning for military and law enforcement personnel, as well as numerous civilian activities such as earthquake detection and monitoring, subsurface geologic probing and mineral prospecting using controlled seismic events and precision monitoring systems.
The present invention answers the long-felt need for a seismic detector for lower strain values of about 0.5μ strain that does that does not suffer from the disadvantages, shortcomings and limitations of prior art arrangements by using multiple filter Bragg gratings seismic activity sensors in a simple, low-cost continuous fiber loop or strand. The first step of seismic monitoring is basic detection. To perform basic detection, a highly sensitive sensor is especially important to collect all the seismic waves efficiently. The fiber Bragg gratings seismic activity sensor and sensor array provide a number of advantages that are well suited for this purpose, and in general for military sensing and detection systems.